DE 199 39 908 A1 discloses a fuel tank in which a closure flap is arranged pivotably at the end of the fuel tank filling pipe, being the end which is towards the interior of the fuel tank and which projects into the interior of the tank itself. The closure flap is movable between a closed position in which it shuts off the through-flow cross-section of the filling pipe and an open position in which it opens that cross-section. The closure flap is urged in the direction of the closed position by a spring and is pivotable towards its open position by a flow of fuel as it flows into the fuel tank. The closure flap is intended to prevent what is referred to as the spit-back effect in the process of filling the fuel tank, for example in the case of a motor vehicle.
That spit-back effect is caused by the internal pressure in the fuel tank abruptly rising towards the end of a tank filling process, because of actuation of a tank filling vent valve. That increase in pressure in the interior of the tank causes fuel to pass upwardly in the filling pipe and thus causes the tank refueling gun to be switched off automatically. The pressure conditions in the interior of the fuel tank at the end of the refueling phase however can be such that fuel spits back out of the filling pipe. It has been found that such a spit-back effect occurs in particular when filling a fuel tank with diesel fuel as diesel fuel has a tendency to foam up as it is flowing into the tank.
The closure flap constituting a check valve as described in DE 199 39 908 A1 is intended to perform the function, as set forth therein, of preventing fuel from escaping from the fuel tank through the filling pipe in the situation where the portion of the filling pipe, which is at the outside of the tank, is damaged as by being holed or completely severed in an accident. On the one hand, that arrangement is intended to afford sealing integrity in relation to a column of fuel rising upwardly within the filling pipe from the interior of the tank, while on the other hand the check flap arrangement is intended to be easy to open under the pressure of the fuel as it flows into the tank in a refueling process.
To adequately seal off the closure flap, DE 199 39 908 A1 proposes a sealing ring of elastomer material, which is adapted to co-operate with a sealing surface having a substantially line-shaped sealing edge, on the end of the filling pipe. The co-operation of the line-shaped sealing edge and the sealing element means that a comparatively high closure force is required to achieve sealing integrity at that location. As indicated hereinbefore however it is desirable to afford a reliable sealing effect while involving a low level of closure flap closing force, that is to say a closing force which allows the closure flap to open relatively easily under the pressure of the incoming flow of fuel. Because of the comparatively high pressure in relation to surface area as between the sealing element and the sealing edge, which is required in the above-discussed arrangement to afford sealing integrity, a reliable sealing effect with a low level of closing force is only possible if the spring which is operable to hold the check flap in the closed position is of a suitable design. That however has an adverse effect on the ease of movement of the closure flap and the ease of opening same, under the force of the incoming flow of fuel.
In another discharge nozzle for a filling pipe of a fuel tank such as for a motor vehicle as disclosed in DE 200 01 334 U1, comprising a check flap held in a closed position under a spring loading and caused to bear pivotably against the mouth opening of the nozzle, with the check flap being sealed in relation to the nozzle by way of a sealing element with a peripherally extending sealing lip, the structure involved is designed to address the problem of sealing integrity of the check flap, using a sealing ring which is arranged at the end of the discharge nozzle and which has the sealing lip which is directed radially inwardly and engages over the end face of the discharge nozzle. It is stated therein that the structure of that arrangement suffers from the disadvantage that, in the presence of fuel, the inside edge of the sealing lip adopts a corrugated or wavy configuration in the peripheral direction and accordingly lifts off in part from the check flap so that the sealing lip does not bear in completely sealing relationship against the check flap. That publication therefore proposes the provision of a sealing limb at the free inside edge of the sealing lip, the sealing limb extending axially in a direction towards the check flap which then bears against the sealing lip.
It has been found in practice that such a design configuration for the discharge nozzle is not of an optimum nature. The structure involved therein only partially counteracts the corrugation effect for the seal in the presence of fuel, for example fuel which has a high proportion of alcohol in it. The closure pressure of the check flap or valve flap must be set to be so slight that easy opening of the flap is guaranteed, even when the flow of fuel by volume passing into the tank through the discharge nozzle is comparatively slight. Depending on the respective kind of fuel and the period for which the seal is immersed in the fuel the seal can nonetheless suffer from the corrugation effect so that there is no longer any guarantee of sealing integrity by virtue of line contact between the sealing surfaces.
In addition the pressure of a column of liquid which occurs at the check flap acts both axially and also radially so that it is not impossible that the radially acting liquid pressure can still lift off the seal. At the least the radial pressure of the liquid acts against the closing pressure of the check flap, which is produced by the spring, so that for that reason also it is not possible to guarantee that the sealing element bears in line sealing contact against the sealing surface over a prolonged period of time.